1. Field of the Invention
The present invention relates to functional film networks, and in particular to sequentially deposited radio frequency plasma film layers having an open network structure, thereby increasing interstitial spacing between plasma film layers and providing access to functional groups contained therein.
2. Previous Art
The surfaces of polymeric, metal and ceramic materials are important in many applications. Often these surfaces must be modified for a specific use. For example, surfaces of medical devices implanted in the body must have biocompatible surfaces.
Different methods are generally employed to modify the surfaces of polymers, as opposed to metal and ceramic surfaces. Several conventional methods of surface modification employ wet chemical processes. Most recently developed are energetic methods of surface modification. Each of these methods for each type of material is discussed below.
Wet chemical surface modification of metals and ceramics is accomplished either by forming composites where the metals and ceramics are blended with matrix resin, or by coating these substances with organic coatings.
A typical wet chemical approach of surface modification of polymeric materials employs acids to etch and oxidize the surface. Other approaches employ solvent swelling and penetration of topical coatings into the swollen surfaces. Upon evaporation of the solvent, the coating is incorporated into the top layer of the polymeric article.
There are many problems associated with use of solvents and other wet chemical methods for modifying surfaces. For example, the use of wet chemical methods to modify surfaces can take several steps to accomplish. The chemicals used are often messy, corrosive and toxic to both humans and the environment. There are often many steps, such as the application reaction, rinsing, and neutralization. It is not easy to change steps if sequentially applying several chemicals. Not all surface areas of the material to be modified are accessible to the wet chemicals, such as blind vias and other hidden surfaces. The monomers used must be reactive. Yields are low and solvents can leave residues on the surface leading to contamination of the surface. Additionally, some wet chemical methods can also damage the surface that one is attempting to modify.
Surface composition of polymeric materials is commonly modified by blending additives into the bulk polymer before fabrication and allowing surface active agents to migrate to the surface. The end groups of the polymer chain can also be modified with specific functional groups. Changes to the bulk of the polymer are thus minimized. The added mobility of the end groups relative to the polymer backbone appear to facilitate self-assembly of the molecular overlayers by the surface active end blocks.
A major drawback of this method of surface modification is that there is a limit to the chemical functional density that can be incorporated without significantly altering the basic nature of the material.
Energetic processes (i.e., plasma) for surface modification of polymeric materials have also been gaining acceptance in a number of industries. In plasma modification, the bulk properties of the original polymer are retained while chemically changing only the top 20 .ANG. of the surface. Polymers such as polypropylene, polystyrene, polyester, Teflon.RTM. and other commercially available polymers have been modified using this method. For example, a polystyrene material that normally does not contain nitrogen can be modified using ammonia gas ionized in a radio frequency (RF) field. This method commonly employs a vacuum chamber, means for introducing a reactive gas such as oxygen, ammonia or nitrous oxide into the chamber and RF energy as tools in the modification process.
In plasma surface modification, the gas employed for modifying the surface of the polymer is introduced into the vacuum chamber containing the surface to be modified. The gas is ionized using RF energy and this ionized gas interacts with the surface of the material. Ionized gases contain a mixture of highly reactive chemical species that include free radicals, electrons, ions and metastable reactive species. These species easily break the chemical bonds on the surface of polymeric materials and substitute the desired chemical groups on the surface of the material. In this manner carbonyl, carboxylic acid, hydroxy, and amine functional groups have been incorporated into and hence become a part of polymeric surfaces.
The design of the reaction chamber, the distribution of power, the excitation frequency, and the gas dynamics are critical factors influencing the properties and efficiency of plasma reactions. Extensive work has been published that shows a direct correlation between excitation frequency and plasma reactivity.
Unlike polymeric materials, metals and ceramics do not contain bonds that can be easily broken. Plasma film deposition offers a means for modifying the surfaces of such materials. In this process monomers consisting of polyatomic molecules are typically ionized using RF energy.
Using plasma polymerization (or plasma film deposition), functional groups can be incorporated into or deposited on any surface, including polymers, metals, ceramics and composites. The films deposited using plasma polymerization are compositionally very different from the polymers formed in bulk processes of polymeric materials using these same monomers. Materials such as methane, propane, and other saturated hydrocarbons are commonly employed to deposit plasma polymerized films on metals and ceramics. Additionally, the film can be comprised of amines, acids, methacrylates, glycidyls or mixtures such as methane and amine, or methane and acid.
When depositing functional films on surfaces using plasma film deposition, the functional density in most cases is limited to that achieved by a monolayer. For example, 11 atom % nitrogen in films deposited from diaminocyclohexane on polystyrene was reported in Clinical Materials 11 (1992). This concentration equates to a concentration of 1 nmoles/cm.sup.2 of primary amines on the surface or a coverage of one monolayer of amine on the surface.
The difficulty with most single monolayers of functional density is that there are a limited number of chemically reactive sites that are available for interaction with the desired coating material such as a biomolecule or the matrix resin of a composite. When the number of functional groups available on the surface of a substrate is limited, the benefits that can be achieved are also limited. In the case of composites, the number of locations where the matrix resin is bonded to the reinforcing materials is limited and the ultimate strength of the composite material is also limited. In the case of biomolecule attachment, lower functional densities decrease the amount of these materials that can be anchored on the surface. Often attachment of more than one biomolecule is desired to facilitate multiple performance attributes. In these cases the amount of any given material that can be attached is decreased and may be below the minimum threshold needed for the desired performance.
Plasma polymerized films have also been deposited using acrylic acid which produces films with a high density of functional groups. The density is achieved by building a linear polymer of acrylic acid on the surface. Additionally, soft plasma or pulse plasma has been employed with variable duty cycle to preserve the functional groups of films during deposition using plasma polymerization. In addition to only leaving a single monomer layer deposited, these methods also depend on building long linear chains anchored to the surface to generate the high functional densities that are desired.
Further, in plasma deposition, the energy per mole of monomer determines the number of bonds broken. At high power and low monomer concentration (hard plasma) more of the bonds are broken and less of the functional character is retained.
It is known that the power applied, the frequency of the pulse, and the duty cycle can all be varied to preserve the functional nature of the deposited film. Indeed, it has been found that by using high power coupled with a low duty cycle, a higher portion of the functional nature of the deposited film is maintained. A major drawback of these methods is that the films that are deposited are mechanically weak and can be easily abraded away. Furthermore, during plasma film deposition, there are two competing processes that occur. One is the deposition of the film and the other is the ablation of the film being deposited. The degree to which one process predominates is a function of both the process conditions being employed and the chemical nature of the film being deposited. In an attempt to build sufficient functional density on the surface using plasma polymerization, there is also an inherent risk that some of the film being deposited will be ablated away due to the process conditions that need to be employed.
Even if sufficiently long chains of reactive groups could be deposited, the groups at the lower regions of the film may not be as easily accessible for interactions with coating materials as is desired. For example, in films deposited from allylamine, it has been found that a primary amine concentration on the surface is not as high as would be expected from the nitrogen content of the surface measured by ESCA. It has been concluded that perhaps some of these functional groups were buried and not accessible on the surface for reaction with the derivatizing reagents used in their analysis.
Finally, star polymers have been created employing wet chemical methods. For example, the synthesis of star polymers have been reported after reacting multifunctional isocyanates with glycols.
In U.S. Pat. Nos. 4,507,466; 4,588,120; 4,568,737; 4,587,329; and 4,694,064, herein incorporated by reference, Tomalia, et al. disclose synthesis of giant star polymers commonly referred to as "dendrimers". In the noted patents, sequential reactions of methylacrylate and ethylenediamine are achieved employing methanol as a solvent. Star polymers offer several advantages, namely, a network structure that provides physical strength and the ability to provide high chemical functional densities.
There are several problems associated with star polymers. First, the conventional method of building molecules from the core produces only small quantities of star polymers and requires several days to accomplish. Second, large scale synthetic methods remain to be developed.
Additionally, in order for star polymers (and dendrimers) to be useful in modifying surfaces of materials, star polymers must be anchored to reactive sites on surfaces using reactive cores as attachment points. This type of anchoring has many problems. For example, it is difficult to attach dendrimers to surfaces because the anchoring point of the core is located in the center of the star. Thus, anchoring can only occur through a reactive group on the periphery of the dendrimer. Even in these cases the substrate to which the dendrimer is attached must be modified by some means to allow attachment.
Steric hindrance of the star also limits the amount of dendrimers that can be attached to a surface. Additionally, it is easy to break this single attachment.
In biomedical applications, for example, a stent or other object placed in the body, the medical devices must have the biomedically active agent fixed to and completely cover the surface. Dendrimers provide space between each attachment, leaving substrate surface areas exposed to body fluids.
Most plasma processing techniques employ the deposition of functional groups on the surface as the end point of their process rather than as an intermediate link in an ultimate structure. Therefore, practitioners employ conventional materials such as oxygen, ammonia and other such materials to deposit functional groups on the surface. For example, in U.S. Pat. No. 5,342,693, herein incorporated by reference, a glassy film is deposited using siloxanes ionized in a plasma. Using the methods of plasma surface modification, ammonia is then used to provide amine functional groups on the surface. Other materials are subsequently attached to this functional group using wet chemical methods.
Therefore, what is needed is (i) a sequentially deposited film network comprising several RF plasma layers and having a strong interface, (ii) a method to provide high functional density film networks with controllable amounts of crosslinking for accessible functional groups, and (iii) means for providing large scale RF plasma deposition that can be accomplished in a relatively short time without employing wet chemical methods.